Title: Uranyl and Mercuric Ions Mobility in Soil
Client Challenge
This client requested research of media options for the control of select toxic and radioactive metals in water and soil media.
MCLinc Approach
MCLinc researchers utilized data from equilibrium batch tests, treatment canisters, and soil column studies to investigate the efficacy of ground biogenic apatite for the control of select toxic and radioactive metals in water and soil media. When used as a soil amendment, apatite is an effective medium for in situ stabilization of a wide variety of priority cationic metal contaminants (including uranium). The affinity for U is especially strong; uranium appears to initially sorb to the apatite surface, rather than undergoing homogeneous precipitation from solution phase as an insoluble phosphate salt. At high U loading on apatite (several wt%), phase transitions are noted, with formation of highly insoluble crystalline minerals of the autunite group (M(UO2)2(PO4)2·nH2O). MCLinc performed testing of biogenic apatite laden with select metals (Uranium, Cadmium, Lead, Mercury) and demonstrated that these metals did not leach from the apatite host at concentrations above the criteria defined in the Toxicity Characteristic Leaching Procedure (TCLP) and other critical performance assessments. Two series of tests were performed: (a) elution of metals from contaminated soil by clean groundwater proxy, and (b) elution of select toxic metals fed to clean soil.
Outcome
In the absence of added apatite, uranyl and mercuric ions are relatively mobile in soil. Retention of these metals is greatly enhanced by apatite amendment. In general, apatite amendment decreases the exchangeable fraction for toxic cations in soil, transforming the metals into more leach-resistant phase.